Use synthetic bold/italic terminology consistently

We currently use the terms fake bold/italic and synthetic bold/
italic interchangeably. This change replaces all uses with the more
descriptive synthetic bold/italic terminology.

BUG=335768
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/138273018

git-svn-id: svn://svn.chromium.org/blink/trunk@165354 bbb929c8-8fbe-4397-9dbb-9b2b20218538